I've just discovered https://www.gumlet.com/tool/browser-capabilities-checker/ and it shows that both HEVC codecs are not supported by my instance of LibreWolf
LibreWolf
Welcome to the official community for LibreWolf.
LibreWolf is designed to increase protection against tracking and fingerprinting techniques, while also including a few security improvements. LibreWolf also aims to remove all the telemetry, data collection and annoyances, as well as disabling anti-freedom features like DRM. If you have any question please visit our FAQ first: https://librewolf.net/docs/faq/
To learn more or to download the browser visit the website: https://librewolf.net/
If you want to contribute head over to our Codeberg: https://codeberg.org/librewolf
I think HEVC is not supported by any version of firefox, because that codec has a garbage license and firefox can't afford it
Hardware-accelerated playback of HEVC video content is now supported on Windows.
Hardware-accelerated playback of HEVC video content is now supported on macOS.
137.0a1 Firefox nightly
Warning: Features listed here may or may not make a final release of Firefox.
Support HEVC playback on Linux.
oh, something has changed! that's interesting.